Ventricular fibrillation is the rapid uncoordinated fluttering contractions of the ventricles of the heart resulting in loss of synchronization between the heartbeat and pulse beat. Unless the victim is electrically defibrillated within the first two minutes of onset of the ventricular fibrillation, complementary therapy is indicated including conventional cardiopulmonary resuscitation (CPR) with compression of the chest, drug treatment, and occasionally open heart message. The use of electrical defibrillation is often dependent upon the identification of the situation by emergency personnel who work under adverse conditions and time pressures. Inappropriate timing of defibrillation is harmful because it of itself produces myocardial injury reducing the likelihood of ultimate survival.
Current resuscitation methods are constrained in part by the lack of practical and reliable real time monitors of the efficacy of electrical defibrillation interventions. This is especially pertinent to current cardiopulmonary resuscitation (CPR) practices in which precordial compression is interrupted for repetitive attempts at electrical defibrillation during hands-off intervals. When critical levels of coronary perfusion cannot be maintained, the electrical defibrillation attempt predictably fails, Niemann, J. T., et al., "Treatment of Prolonged Ventricular Fibrillation. Immediate Countershock Versus High-dose Epinephrine and CPR Preceding Countershock," Circulation, 85:281-287, 1992. This issue is also of practical moment with respect to programming automated external defibrillators which require even greater hands-off intervals.
Studies of human victims of cardiac arrest have previously suggested that the amplitude of ventricular fibrillation (VF) waveforms may predict the outcome of a defibrillation attempt. Significantly greater VF amplitudes are associated with correspondingly greater likelihood of restoring spontaneous circulation, Weaver, W. D., et al., "Amplitude of Ventricular Fibrillation Waveform and Outcome After Cardiac Arrest," Annals of Internal Medicine, 102:53-55, 1985; Stults, K. R., et al., "Ventricular Fibrillation Amplitude Predicts Ability to Defibrillate," Journal of the American College of Cardiology, 9:152A (abstract only), 1987; Dalzell, G. W., Adgey, A. A., "Determinants of Successful Transthoracic Defibrillation and Outcome in Ventricular Fibrillation," British Heart Journal, 65:311-316, 1991; and, Callaham, M., et al., "Prehospital Cardiac Arrest Treated by Urban First-responders; Profile of Patient Response and Prediction of Outcome by Ventricular Fibrillation Waveform," Annals of Emergency Medicine, 22:1664-1667, 1993. In addition to the ventricular fibrillation (VF) amplitude, the median frequency of VF obtained by Fourier analysis of VF waveforms serves as a predictor of the success of electrical defibrillation in swine, Brown, C. G., et al., "Median Frequency--A New Parameter for Predicting Defibrillation Success Rate," Annals of Emergency Medicine, 20:787-789, 1991. Also, the median frequency of VF serves as a predictor in humans, Strohmenger, H. U., et al., "Frequency of Ventricular fibrillation as Predictor of Defibrillation Success during Cardiac Surgery," Anesthesiology Analogs, 79:434-438, 1994. High frequency of ventricular fibrillation (VF) wavelets are associated with significantly greater success of defibrillation. The frequency of VF is also related to the duration of untreated VF, Brown, C. G., et al., "Estimating the Duration of Ventricular Fibrillation,"Annals of Emergency Medicine, 18:1181-1185, 1989; Dzwonczyk, R., et al., "The Median Frequency of ECG During Ventricular Fibrillation: Its Use in an Algorithm for Estimating the Duration of Cardiac Arrest," IEEE Transactions in Biomedical Engineering, 37:640-646, 1990; Martin, D. R., et al., "Frequency Analysis of the Human and Swine Electrocardiogram During Ventricular Fibrillation," Resuscitation, 22:85-91, 1991; and Brown, C. G., et al., "Physiologic Measurement of Ventricular Fibrillation ECG Signal: Estimating the Duration of Ventricular Fibrillation," Annals of Emergency Medicine, 22:70-74, 1993. Both ventricular fibrillation (VF) amplitude and frequency have therefore emerged as potentially promising real time and noninvasive measurements for guiding resuscitation interventions.
U.S. Pat. No. 5,077,667, issued Dec. 31, 1991, to Charles G. Brown and Roger Dzwonczyk entitled "Measurement of the Approximate Elapsed Time of Ventricular Fibrillation and Monitoring the Response of the Heart to Therapy" describes in detail a method for using the frequency of ventricular fibrillation (VF). The approximate elapsed time since the onset of VF is detected from an analog electrocardiogram signal. The signal is digitized for a time interval of four seconds to obtain a data set of time domain samples. These time domain samples are Fourier transformed to a frequency domain spectrum and the median frequency which bisects the energy of the power spectrum is detected. That median frequency is then compared to a pattern of experimentally obtained median frequency data as represented by a mathematical algorithm to calculate the estimated time from the onset of ventricular fibrillation. This frequency parameter can also be used to evaluate the response to therapy during ventricular fibrillation and CPR, as well as estimate the most appropriate time to defibrillate a subject following various pharmacologic and mechanical interventions.
U.S. Pat. Nos. 5,571,142 and 5,683,424, issued Nov. 5, 1996, and Nov. 4, 1997, respectively, also to Brown and Dzwonczyk, entitled "Non-invasive Monitoring and Treatment of Subjects in Cardiac Arrest Using ECG Parameters Predictive of Outcome" describe in detail methods and apparatus for determining the metabolic state of the myocardium of a subject in ventricular fibrillation or asystole and/or for guiding therapeutic interventions using the frequency of ventricular fibrillation (VF). Electrocardiographic signals of the subject are transformed to a frequency domain power spectrum, and at least one frequency parameter is monitored and processed to a value predictive of a clinically relevant cardiac arrest outcome. In the preferred embodiment, centroid frequency and/or peak power frequency of the power spectrum are monitored.
U.S. Pat. No. 5,643,325, issued Jul. 1, 1997, to Hrayr S. Karagueuzian, et al., entitled "Defibrillator with Shock Energy Based on EKG Transform" describes another method for using the frequency of ventricular fibrillation (VF). A phase-plane plot is made of a patient's electrocardiogram. The degree of deterministic chaos in the phase-plane plot is measured by a processor. Analysis of the phase-plane plot may indicate a propensity for fibrillation including both the risk of fibrillation and the actual onset of fibrillation. A second method for detecting a heart disorder comprises examination of a frequency domain transform developed by applying a fast Fourier transform to the EKG of a patient. An automatic defibrillating device delivers a shock which varies in size, at least in part, according to the peak energy disclosed by the fast Fourier transform.
Coronary perfusion pressure is the best single predictor of the success of cardiac resuscitation in animals, Ralston, S. H., et al., "Intrapulmonary Epinephrine During Prolonged Cardiopulmonary Resuscitation: Improved Regional Blood Flow and Resuscitation in Dogs," Annals of Emergency Medicine, 13:79-86, 1984; Halperin, H. R., et al., "Determinants of Blood Flow to Vital Organs During Cardiopulmonary Resuscitation in Dogs," Circulation, 73:539-550, 1986; and Wolfe, J. A., et al., "Physiologic Determinant of Coronary Blood Flow During External Cardiac Massage," Journal of Thoracic Cardiovascular Surgery, 95:523-532, 1988. Also, coronary perfusion pressure is the best single predictor in humans, Paradis, N. A., et al., "Coronary Perfusion Pressure and Return of Spontaneous Circulation in Human Cardiopulmonary Resuscitation," JAMA--Journal of the American Medical Association, 263:1106-1113, 1990. A threshold coronary perfusion pressure of 10 mm Hg is required in pigs for successful defibrillation and restoration of spontaneous circulation, Grundler, W., Weil, M. H., and Rackow, E. C., "Arteriovenous Carbon Dioxide and pH Gradients During Cardiac Arrest," Circulation, 74:1071-1074, 1986; Gudipati, C. V., Weil, M. H., Bisera, J., et al., "Expired Carbon Dioxide: a Noninvasive Monitor of Cardiopulmonary Resuscitation," Circulation, 77:234-239, 1988; and Gazmuri, R. J., von Planta, M., Weil, M. H. et al., "Cardiac Effects of Carbon Dioxide-consuming and Carbon Dioxide-generating Buffers During Cardiopulmonary Resuscitation," Journal of the American College of Cardiology, 5:482-490, 1990. A threshold coronary perfusion pressure of 15 mm Hg is required in humans for successful defibrillation and restoration of spontaneous circulation, Paradis, N. A., et al., supra.
However, there is currently no noninvasive or practical invasive option for measuring coronary perfusion pressure in or out of hospital settings. With the possible exception of end tidal CO.sub.2, no practical alternative has been identified, Falk, J. L., Rackow, E. C., and Weil, M. H., "End-tidal Carbon Dioxide Concentration During Cardiopulmonary Resuscitation," New England Journal of Medicine, 318:607-611, 1988; and Sanders, A. B., et al., "End-tidal Carbon Dioxide Monitoring During Cardiopulmonary Resuscitation. A Prognostic Indicator for Survival," JAMA, 262:1347-1351. But these measurements require extensive instrumentation and sophistication ordinarily available only in experimental situations.
In view of the problems associated with measuring coronary perfusion pressure, electronic analysis of electrocardiographic ventricular fibrillation (VF) signals would appear to be more practical in most real world situations where the availability of specialized equipment is limited. The need for accurate electronic analysis has escalated since the introduction of automatic external defibrillators. Substantial hands-off intervals are mandated during which precordial compression is held in abeyance when the automatic external defibrillator cycles through electrocardiographic analysis, capacitor charge, and capacitor discharge. The process may be repeated for as many as three cycles and consume as long as 82 seconds. During such prolonged hands-off intervals, no cardiac output or coronary blood flow is generated by precordial compression. Such delays are likely to compromise the ultimate success of cardiopulmonary resuscitation and there is evidence of such based on recent studies on a murine model, Sato, Y., Weil, M. H., Sun, S., et al., "Time Limitations Between Stopping Precordial Compression and Defibrillation," Critical Care Medicine, 24 (1):A116 (abstract only), 1996.